The process of aging is fundamental to virtually all multicellular organisms. Recently much progress has been made in defining various environmental and genetic factors that determine an organism's lifespan, including the effect of accumulated cellular damage and the influence of genetically identified "pro-aging" genes. Despite these advances, relatively little is known about the changes that these factors produce at the cell and tissue level to give rise to phenotypes associated with aging. One hypothesis to explain at least some of the changes associated with aging is that changes occur in the adult stem cells that maintain most tissues. In contrast to cells in many tissues that divide a finite number of times and undergo rapid turnover, stem cells exist throughout the lifetime of the organism, and may divide many more times than non-stem cells. Thus they may be particularly susceptible to factors contributing to aging, and phenotypic changes in these cells have a large influence on the phenotype of the tissue. The research proposed examines the effect of aging on stem cells in the Drosophila testis, which presents 3 major advantages for this type of study. 1) In contrast to many other stem cell systems, individual stem cells in the Drosophila testis are readily identifiable within their niche. 2) The Drosophila testis contains two distinct stem cell populations - germline stem cells (GSCs), which give rise to the sperm that will contribute to future generations, and somatic stem cells, which divide to produce cyst cells that will support the development of germ cells. 3) The mechanisms of stem cell function in the Drosophila testis have been extensively studied, and there are well-established tools that can be used to mark and manipulate these cells. Preliminary work has shown that GSCs exhibit a slowdown in cell cycle activity during normal aging, but that this slowing is not seen in long-lived methuselah (mth) mutant flies. This work proposes to first determine by mosaic analysis whether mth function is required in the GSCs themselves, in the somatic niche, or elsewhere in the fly to promote normal aging of GSCs. Mth mutant flies have been shown to be resistant to oxidative stress, and accumulation of oxidative damage has been proposed as a major factor in aging. We will therefore test the hypothesis that accumulation of such damage in GSCs or their environment contributes to cell cycle slowing during aging. This will be done both by preventing accumulation of such damage by overexpressing superoxide dismutase in flies, and but increasing oxidative damage through paraquat treatment. Finally, because germ cells likely contain mechanisms to protect accumulation of aging-related damage, we will compare the activity of GSCs to that of the somatic stem cell population in the testis, using lineage tracing methods similar to what was done previously in the germ line. Relevance: Stem cells play a critical role in multicellular organisms for both tissue homeostasis and injury repair. Understanding the effects of aging on stem cells will aid in development of stem cell- based therapies, and may well allow us to better understand declines in organismal function that occur during aging. [unreadable] [unreadable] [unreadable]